Inductive power transfer systems for use in distributing electrical energy are now well known. These systems typically comprise an elongate primary conductor which is energised at a desired frequency so that a number of tuned “pick-up” or secondary power distribution units which are positioned closely adjacent to the primary conductor may be energised by the magnetic field associated with the primary conductor. Therefore, IPT systems have the advantage that a contactless power distribution system is achieved.
These systems have significant benefits in a number of applications such as lighting for roadways, tunnels, swimming pools and aircraft. In particular these systems are particularly advantageous for use in situations where an unfavourable environment for power distribution may exist. For example, roadways are subject to considerable physical stresses due to large fast moving vehicles. Illuminated road studs that are powered by fixed wiring are difficult to wire in to the roadway and are prone to failure due to stresses from vehicles that can physically move the stud or the adjacent road surface and allow foreign matter such as water or dirt to interfere with the wired connection.
An issue with illuminated road stud IPT systems, where the primary conductor is required to be buried in the road substrate, is that it can be necessary to transfer power over an extended distance or gap from the primary conductor. Although the primary conductor may be buried at a suitable level to enable adequate power transfer when such a system is first installed, roadways often need to be resealed which can extend the distance between the primary conductor and the pick-up. As distance from the primary conductor increases, less may power may be transferred, until such time as the light supplied by the pick-up becomes weak or does not function at all.
A solution to this problem has been proposed in U.S. Pat. No. 6,317,338 which suggests use of an intermediate resonant circuit. This is a resonant circuit which may be placed between the primary conductor and one or more of the secondary pick-ups. In the context of lighting systems, the specification discloses an intermediate resonant circuit which comprises an inductive element and a capacitive element, the values of which are selected to provide resonance at a desired frequency. Therefore, the intermediate resonant circuit is energised by energy derived from the magnetic field associated with the primary conductor. As a result of energisation of the intermediate resonant circuit, a magnetic field is established which in effect extends the field associated with the primary conductor. This allows power to be transferred over an extended distance or gap from the primary conductor.
U.S. Pat. No. 6,317,338 discloses intermediate resonant circuit constructions of conductors that have inductive and capacitive properties that lie parallel to the primary conductor. This is undesirable in many practical applications, for example in a road stud installation a significant section of roadway needs to be cut or excavated to install such a system. This form of circuit presents installation difficulties if primary cables need to be run through an existing cable duct in a structure such as a bridge. Also, the physical construction of such a circuit is difficult to manufacture and is prone to failure. If a road including this known design needs surface reconstruction, then there is a substantial risk that the intermediate resonant circuit will be destroyed. Finally, the known arrangement has the disadvantage that is effectively only intercepts the field from one of the conductors in the primary conductive path.